Any unit that has a negative Land Attack national capability offset will not contribute to obtaining (or blocking) CA.

If you do obtain CA (via other units), the negative LA offset units still benefit from it.

This sounds good. What about on the other side - can minor Axis units count toward preventing the Allies from getting the CA bonus? If you have 2 German tanks and 1 Italian tanks for example, would the Russians need 3 tanks or only 2 to get the CA bonus? Or if you have one of each unit (infantry, tank, tacair, and artillery) - would it matter that one was Italian?

As far as determining whether CA is applicable or not, if the unit has a negative national capability offset for LA just pretend it is not present. It doesn't factor into giving the CA to the attacker nor blocking the CA for the defender. That includes the armor unit count.